The speed control of DC motors has long been a problem as far as energy conservation is concerned when the motor is not operated at "full throttle", but at some varying speed from zero to full rpm. Such examples include electric automobiles or golf carts where gradual application of power and an increasing application of power would be desired as the vehicle begins to move and increase speed or if the vehicle were desired to operate at less than full speed. Further examples of variable speed, DC motor situations include high and low speed heater and air conditioner fans, electric windshield wiper motors, and many others.
In order to vary the speed of a DC motor, by far the most commonly used prior method is to place one or more resistors or a variable resistor in series with the motor and battery or other power source. The torque or speed of the motor is then varied by increasing or varying the number of these resistors or the amount of resistance in the circuit to the motor. The disadvantage of this method is readily apparent as far as wasted energy is concerned because controlling the speed in this manner means that large amounts of the power output of the battery or power source is always being pulled, regardless of the motor speed. If the motor is not operating in full capacity, the remainder of the power generated by the power source is wasted in the control or "drain" resistors in the form of heat. To further compound this situation in some cases it is even necessary to use a companion electric motor to operate a fan to blow cool air across the resistors.
Other speed control methods include rheostatic adjustment of the field current which varies the field strength, however, this also requires full voltage at all times from the power source. The Ward-Leonard system is another speed control technique which utilizes a generator to supply the motor armature and does do away with the armature rheostatic losses. However, this approach is extremely expensive as far as initial expense of added equipment is concerned.